It isn't clothing per se that keeps your body warm. Instead it's a thin pocket of dead air between your skin and your first layer of clothing. I'm a big fan of Patagonia products—sometimes jokingly or dismissively referred to as "Patagucci" for their high price tags, but always worth every penny. Patagonia's R1 pullover has an unusual grid pattern on the inside that creates numerous tiny pockets of air close to the skin. As a result, this moisture-wicking polyester pullover is unusually warm for its weight. It's an excellent insulator. (That's the end of tonight's unpaid advertisement.) Old-time Registered Maine Guides tell stories of having to spend nights in the woods and stuffing their wool underwear with dead leaves to create the same effect. Leaves (or crumpled newspapers) won't keep you warm but they do trap body heat and keep it circulating where it will keep your skin warm.